A police officer holds tape to keep people out from an area at Harvard University in Cambridge, Mass., today. Four buildings on campus were evacuated after campus police received an unconfirmed report that explosives may have been placed inside, interrupting final exams.

By DENISE LAVOIE

Associated Press

CAMBRIDGE, Mass. - Students have been cleared to return to all 4 Harvard campus buildings evacuated over an explosives report.

Students have been told that they may return to the classroom building. Harvard Yard has also re-opened.

Two other buildings, a dorm and another classroom building, reopened earlier. The fourth building, the school's Science Center, just reopened.

A search has yet to turn up any explosives.

An alert went out shortly after 9 a.m. today telling students at the Ivy League school outside Boston to leave the four buildings.

It's exam time at Harvard and some students were just about to start finals when the alert went out.

The FBI is investigating along with Cambridge and university police and other agencies.